The invention relates to a position sensor for sensing the position of a moving part capable of actuating a plunger belonging to the sensor. More specifically the invention discloses a mechanism for orienting the plunger of the sensor.
A position sensor is a device designed to be actuated by a moving part, either because of the shape of its plunger or because of the force required to actuate it.
The position of the moving part is sensed by the mechanical action which the part exerts on the plunger when in contact with it.
Position sensors are usually fitted with miniature switches combining a reliable system of rapid switching with a sealed enclosure that confines the contacts in a neutral gas atmosphere favorable to the switching of low-level circuits up to high currents. The displacement of the plunger by the moving part actuates the miniature switch and closes or opens an electrical contact which signals the presence of the moving part.
The shape of that part of the plunger which comes into contact with the moving part is adapted both to the shape of the moving part whose position is to be detected and to the type of movement of the part. The reason for this is that, depending on the configuration of the mechanical system in which the sensor is inserted, the angle of attack defined between the direction of the movement of the moving part and the axis of the plunger, when contact occurs between the moving part and the plunger, can differ.
For example, if the displacement of the moving part is in the direction of the axis of the plunger, the free end of the plunger can simply be a hemispherical surface giving point contact with the moving part. With this same type of displacement, the free end of the plunger may be fitted with a ball able to pivot in the plunger in order to provide frictionless contact with the mechanical part, if the latter exhibits lateral movement with respect to the plunger axis.
In cases where the moving part exhibits a linear movement more or less at right angles to the plunger axis, the plunger comprises at its free end a roller oriented in the direction of movement of the part, the roller being integral with the plunger and being able to rotate about an axis of rotation perpendicular to the axis of the plunger.
FIG. 1a shows a partial section through a position sensor 10 according to the prior art, comprising a roller 12, a guide 14 that can be rotationally oriented about a first axis XXxe2x80x2 of rotation, in a body 16 and a plunger 18 sliding without rotation in the guide 14 along this first axis XXxe2x80x2.
At the free end of the plunger is the roller 12 which is of circular cylindrical shape and can turn on a spindle or shaft 20 about a second axis YYxe2x80x2 of rotation, this second axis YYxe2x80x2 being colinear with the axis of revolution of the roller, and perpendicular to the first axis XXxe2x80x2.
When a moving part P travelling in a direction Dz approximately perpendicular to the first axis XXxe2x80x2 and to the second axis YYxe2x80x2 contacts the roller 12 on a point of its cylindrical surface situated above the second axis YYxe2x80x2, a first force F1 is exerted by the moving part P on the periphery of the roller. This first force F1, transmitted to the plunger 18 through the shaft 20, produces a component force F2 in the direction of the first axis XXxe2x80x2. This component force F2 causes the plunger 18 to slide down the guide 14, and the roller 12 down two guide grooves 22 situated in the guide 14 on either side of the first axis XXxe2x80x2.
The sliding of the plunger down the guide takes place without rotation, the second axis YYxe2x80x2 of rotation of the roller 12 being maintained in the same direction throughout the sliding of the roller in the guide grooves 22.
As it slides down the guide 14, displaced by the component force F2 towards the interior of the body, the plunger 18 actuates, via a rod 24, an electric microswitch 26. A return spring 28 is compressed during the displacement of the plunger towards the interior of the body. When the moving part P moves away from the roller, the plunger 18 is returned to its initial position by the return spring 28.
The body 16 of the position sensor is mounted on a frame (not shown in FIG. 1a) and is thus immobilized in its position with respect to the direction of displacement of the moving part. As a consequence the roller 12 must be able to be oriented and maintained in its angular position about the first axis XXxe2x80x2 so that the second axis YYxe2x80x2, about which the roller revolves, is essentially perpendicular to the direction Dz of the movement of the moving part whose position is to be detected. Having the roller oriented in the direction of movement of the moving part ensures that the roller turns properly on its spindle and avoids lateral loads during contact with the moving part.
To this end, the lower part of the guide 14 comprises position grooves 30 distributed around its periphery at an angular pitch a about the first axis XXxe2x80x2. At the upper end of the body 16, the sensor possesses a locator 32 which is fixed so that it cannot rotate in the body of the sensor and which is in the form of a washer with a tooth 34 designed to fit into one of the position grooves 30.
The body has a screwthread 35 on its cylindrical outer surface on which a nut 36 is screwed to clamp the guide 14 and the locator 34 firmly to the body 16.
FIG. 1b shows a perspective view of the guide 14 showing the position grooves 30 distributed around the periphery of the guide at-an angular pitch xcex1 of 45 degrees.
An angular position xcex2 of the guide grooves 22, and consequently the angular position of the roller 12 sliding in these grooves with respect to a reference axis ZZxe2x80x2 of the sensor passing through the first axis XXxe2x80x2 and through the middle of the tooth 34 of the locator 32, will consequently be determined by the selection of one of the position grooves 30, which contains the tooth 34.
The drawing of FIG. 1c shows a simplified top-down view of the sensor taken on AAxe2x80x2, where the angular position xcex2 of the guide grooves 22 is approximately 90 degrees with respect to the reference axis ZZxe2x80x2. In this configuration the second axis YYxe2x80x2 of the roller 12 shown in dashes in FIG. 1c will have turned through the angle xcex2 of 90 degrees with respect to its position in FIG. 1a. 
FIG. 2a shows a view of the principle of a position sensor 40 according to the prior art showing another mechanism for the angular positioning of a plunger 42 with respect to a fixed body 44 of the sensor.
The position sensor 40 comprises, as in the case of the sensor of FIG. 1a, a guide 46 that can be oriented rotationally about the first axis XXxe2x80x2 in the body 44, with the plunger 42 sliding without rotation in the guide 46 along this first axis XXxe2x80x2.
In this embodiment the guide 46 has first holes 48 distributed radially with an angular pitch xcex1 about the first axis XXxe2x80x2. The axes of the holes all lie in the same plane perpendicular to the first axis XXxe2x80x2, and each hole 48 can be lined up with the open end of a second hole 50 in the periphery of the body 44.
A locator in the form of a flexible circular collar 52 encircles the body 44 of the sensor around its periphery. FIG. 2b shows the collar 52 with a short rod 54 in its center which can be inserted into the second hole 50 of the body and into one of the first holes 48-1 of the guide lined up with the first. Selecting one of the second holes 48-2 and keeping it lined up with the first hole by means of the rod 54 ensures an angularly position xcex2 of the guide with respect to the sensor body.
The different mechanisms of angular positioning for sensors in the prior art have drawbacks as follows.
In operation, the sensor suffers impacts when contact occurs between the moving part and the roller. These repeated impacts produce vibrations in the sensor with the risk of loosening the locator from the sensor body. These vibrations can for example shake the collar 52 free from the sensor shown in FIG. 2a or slacken the nut 36 of the sensor shown in FIG. 1a. 
The loosening of the locator and the failure to keep the guide in position in the sensor body means that the roller is no longer oriented and the position sensing function is therefore no longer performed, with the consequences which this can engender in systems requiring a high degree of reliability of operation.
It is an object of the invention to reduce the problems of the prior art by proposing a position sensor for sensing the position of a moving part comprising:
a guide that is rotationally orientable in a body about a first axis XXxe2x80x2 of rotation, and a plunger sliding without rotation in the guide along this axis XXxe2x80x2;
a locator that prevents rotation of the guide in the body about the first axis of rotation, in a given angular position, characterized in that the locator is fixed nonremovably in the body and in that it possesses either several tabs spaced regularly about the guide and capable of fitting into one position groove to immobilize the guide or several position grooves in which one tab can fit to immobilize the guide.
In one embodiment of the position sensor according to the invention, the tab or tabs are designed to be bent to enable at least one tab to be inserted into a position groove of the guide, in order to prevent rotation of the guide in the body.
In a first variant of the sensor according to the invention, the locator possesses several tabs, two of these tabs bent, one into each of two position grooves of the guide thus preventing the guide from rotating in the body.
In another variant of the position sensor, the locator possesses one bendable tab and the guide several position grooves, the angular position of the guide being determined by the choice of one of the position grooves containing the single bent tab.